High temperature molten salt thermal electrochemical cell

ABSTRACT

The use of cobalt oxide (Co 3  O 4 ) in a thermal cell as the cathodeaterial results in a thermal cell with higher open circuit voltage than the present used iron disulfide cathodes.

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto us of any royalty thereon.

This invention relates in general to a high temperature molten saltthermal electrochemical cell and in particular to such a cell includingcobalt oxide (Co₃ O₄) as the cathode material.

BACKGROUND OF THE INVENTION

High temperature molten salt thermal electrochemical cells are widelyused as power sources for projectiles, rockets, bombs, mines, missiles,decoys, jammers and torpedoes. They are also used as fuses. Thermalelectrochemical cells are reserve-type cells that can be activated byheating with a pyrotechnic heat source such as zirconium and bariumchromate powders or mixtures of iron powder and potassium perchlorate.Early thermal batteries employed magnesium or calcium as the anode, aneutectic mixture of lithium chloride and potassium chloride as theelectrolyte and cathodes of metal oxides such as copper oxide (CuO),iron oxide (Fe₂ O₃), vanadium oxide (V₂ O₅), tungsten oxide (WO₃) andmetal chromates such as calcium chromate and potassium chromate. Morerecent thermal electrochemical cells use alloys of lithium such aslithium-aluminum, lithium-boron and lithium-silicon as anodes, aneutectic mixture of lithium chloride and potassium chloride as theelectrolyte and iron disulfide as the cathode material. The use oflithium alloys as anodes instead of calcium and magnesium has resultedin higher energy densities. Further, the lithium alloy/iron disulfidethermal electrochemical cell has demonstrated improved performance overthe older magnesium or calcium thermal electrochemical cells. Thelithium-alloy/iron disulfide electrochemical cell is now widely used forvarious military applications as mentioned above and has an open circuitvoltage of about 1.7 volts.

It would still be more desirable, however, for the thermalelectrochemical cell to have an even higher open circuit voltage, agreater energy density, and to exhibit flat discharge plateaus underload at voltages consistently above 2.0 volts.

SUMMARY OF THE INVENTION

The general object of this invention is to provide an improved hightemperature molten salt thermal electrochemical cell. A more particularobject of the invention is to provide such a cell that will be improvedover the presently widely used lithium alloy/iron disulfideelectrochemical cell.

It has now been found that the aforementioned objects can be attained bythe use of a new cathode material namely cobalt oxide (Co₃ O₄) inconjunction with a lithium alloy as the anode and an eutectic mixture oflithium chloride and potassium chloride (m.p. 352° C.) as theelectrolyte. The cell is operated at 400°-500° C. and exhibits flatdischarge plateaus under load at voltages consistently above 2.0 volts.

More particularly, the electrochemical cell of the invention includes alithium-aluminum (48 atomic % Li) alloy as the anode, a eutectic mixtureof lithium chloride (59 mole %) and potassium chloride (41 mole %) witha melting point of 352° C. as the electrolyte and cobalt oxide (Co₃ O₄)as the cathode material. The cell is fabricated by stacking 0.5 inchdiameter pellets of the anode, separator material containing theelectrolyte and the cathode in between disks of molybedenum currentcollectors. The cathode pellet is prepared by pressing 0.3 gram ofcobalt oxide (Co₃ O₄) to a pressure of 2000 pounds using an 0.5 inchdie. The separator disk is prepared by pressing in a similar manner a0.6 gram mixture of 35 weight percent magnesium oxide and 65 weightpercent lithium chloride-potassium chloride eutectic electrolyte. Theanode pellet is prepared similarly by pressing a 0.3 gram mixture of 20weight percent lithium 80 weight percent lithium-aluminium alloy to 1500pounds pressure. Due to the hygroscopic nature of the various cellcomponents, the pellets are prepared inside a dry box with a moisturecontent of less than 10 ppm. The pellets are stacked in a jig and placedinside a sealed glass cell. The jig uses a high temperature stainlesssteel compression string that maintains a constant pressure on thepellets. Thus, when the temperature of the cell is raised to the workingtemperature of 400°-500° C., a slight shrinkage of pellets occurs due tothe melting of the electrolyte and the use of the stainless steelcompression spring allows the maintenance of the electrical contact ofthe electrode pellets with the current collector. The cell is operatedat 400°-500° C. in a flowing atmosphere of high purity dry argon gas inorder to maintain anhydrous conditions because of the hygoscopic natureof cell components.

DESCRIPTION OF THE DRAWING

FIG. 1 shows a cross sectional view of a high temperature molten saltthermal electrochemical cell according to the invention;

FIGS. 2 and 3 show typical discharge curves of the cell of FIG. 1 undera drain of 2 mA/cm².

Referring to FIG. 1, the cell, 10, includes lithium-aluminum alloy anode12, a cobalt oxide cathode 14 spaced from the anode, 12, and aseparator, 16, including an eutectic mixture of lithium chloride andpotassium chloride as the electrolyte positioned in the spaces betweensaid anode, 12, and said cathode, 14. A positive molybdenum currentcollector, 18 is in contact with cathode, 14, and a negative molybdenumcurrent collector, 20 is in contact with anode, 12.

The cell exhibits an open circuit voltage of 2.308 volts and an averagevoltage plateau of 2.24 volts. The FIGS. 2 and 3 correspond to adischarge of the new electrochemical cell at a current density of 2mA/cm². These cells may be discharged at current densities in excess of100 mA/cm² which are generally encountered in thermal batteries. Thehigher voltage obtained of 2.3 volts with this cell as compared to thevoltage of 1.7 volts as obtained with the lithium-aluminium/irondisulfide cell will translate into higher energy densities for the newthermal electrochemical cell.

We wish it to be understood that we do not desire to be limited to theexact details of construction shown and described for obviousmodifications will occur to a person skilled in the art.

What is claimed is:
 1. A high temperature molten salt thermalelectrochemical cell including a lithium-aluminum alloy as the anode, acathode of cobalt oxide (Co₃ O₄) spaced from said anode, and a separatorincluding lithium chloride-potasium chloride eutectic electrolyte in thespace between said anode and said cathode.
 2. A high temperature moltensalt thermal cell according to claim 1 wherein the lithium-aluminiumalloy of the anode includes 48 atomic percent of lithium.
 3. A hightemperature molten salt thermal cell according to claim 1 wherein thelithium chloride-potassium chloride eutectic electrolyte is 59 molepercent lithium chloride 41 mole percent potassium chloride.
 4. A hightemperature molten salt thermal electrochemical cell including alithium-aluminium alloy of 48 atomic percent of lithium as the anode,.acathode of cobalt oxide (Co₃ O₄) spaced from said anode, and a separatorincluding 59 mole percent lithium chloride 41 mole percent potassiumchloride eutectic electrolyte in the space between said anode and saidcathode.
 5. A high temperature molten salt thermal electrochemical cellaccording to claim 4 wherein the cell is fabricated by stacking pelletsof the anode, separator material containing the electrolyte and thecathode in between disks of molybdenum current collector.